Thermal treatment of cancer (hyperthermia) is one of the cancer treatments to kill cancer cells using the character of the cells themselves. From past studies, it is known that the survival rate of cancer cells drastically declines when they are heated to 42-45°C. In this laboratory, we have developed thin antennas to heat only tumor parts to a suitable temperature for treatment by using the microwave thermal effect on a human body. Figure A shows the coaxial-slot antenna, one of the thin antennas developed in this laboratory. The antenna, with a diameter of about 1.5 mm, enables minimally invasive treatment. Figure B shows scene of the clinical treatment carried out with the Tokyo Dental College Ichikawa General Hospital Radiology Department. We confirmed the effectiveness through a number of clinical treatments.